Soy bean milk has been generally produced by heating soy beans in hot water of above 98.degree. C. for several minutes, grinding the heated soy beans together with the hot water with deactivation of lipoxidase, and then separating the liquid from the ground material.
In addition, in the production of soy bean milk, there have been made various attempts to eliminate the lipoxidase which cause the raw odor of soy beans which can be sensed when drinking soy bean milk.
However, it has also been revealed that there is a large amount of the trypsin inhibitor in soy beans, and in some cases, the activity of the trypsin in the stomach is decreased, resulting in indigestion of the protein of soy beans.
In addition, it has been also found that maintaining the ground soy beans at a temperature of 95.degree. C. or more is required to deactivate the trypsin inhibitor together with the lipoxidase.
However, it has become a problem in producing soy bean milk that as the temperature in the grinding process for soy beans is raised as described above in order to deactivate the enzyme, the protein extraction rate and the yield of soy protein are lowered increasingly.